Laser diode, optical disk device and optical pickup

ABSTRACT

A laser diode capable of performing self-pulsation operation, and capable of sufficiently reducing the coherence of laser light and stably obtaining low-noise laser light is provided. The laser diode includes: a laser chip including at least one laser stripe which extends in a resonator length direction between a first end surface and a second end surface opposed to each other, in which the laser stripe includes a gain region and a saturable absorption region in the resonator length direction, and the width of the laser stripe in the saturable absorption region is larger than the width of the laser stripe in the gain region.

RELATED APPLICATION DATA

This application is a division of U.S. application Ser. No. 12/502,564,filed Jul. 14, 2009, now allowed. This application claims the benefit ofpriority to Japanese Patent Application No. 2008-189265, filed Jul. 23,2008 in the Japanese Patent Office. Both of these prior applications areincorporated herein by reference to the extent permitted by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser diode, an optical disk deviceand an optical pick-up, and more specifically to, for example, a laserdiode suitably used as a low-noise light source, and an optical deviceand an optical pick-up each of which uses the laser diode as a lightsource.

2. Description of the Related Art

Laser diodes are used as light sources of optical disk devices whichread information from optical disks such as CDs (compact discs) and DVDs(digital versatile discs). In such a laser diode, light reflected by anoptical disk to be returned to the laser diode, that is, so-calledfeedback light disturbs the oscillation state of the laser diode,thereby to cause noises. As a heretofore known technique of reducingfeedback light-induced noises, a laser diode performing self-pulsationoperation is effective. The principle of such a laser diode is to reducethe coherence of laser light to prevent disturbance of the laser diodecaused by feedback light.

As a laser diode intended for self-pulsation operation, a two-electrodelaser diode is known as described in, for example, Japanese UnexaminedPatent Application Publication No. 2004-186678 and V. Z. Tronciu et al.,Opt. Commit 235 (2004) 409-414. FIGS. 9A and 9B illustrate an example ofthe configuration of a two-electrode laser diode in related art. In thiscase, FIG. 9A illustrates a plan view, and FIG. 9B illustrates asectional view taken along line X-X of FIG. 9A.

As illustrated in FIGS. 9A and 9B, the two-electrode laser diodeincludes a laser stripe (a waveguide) 101 which extends throughout thelength in a resonator length direction between a pair of parallel endsurfaces 100 a and 100 b opposed to each other of a rectangular laserchip 100. The laser stripe 101 has a uniform width throughout itslength. The laser chip 100 includes a semiconductor layer 103 forming alaser structure on an electrically conductive semiconductor substrate102. The semiconductor layer 103 includes an active layer as well as ann-side cladding layer, a p-side cladding layer or the like (notillustrated). A section on a side close to the end surface 100 a of thelaser stripe 101 is a gain region 104, and a section on a side close tothe end surface 100 b is a saturable absorption region 105. The gainregion 104 is formed so as to have a larger length than that of thesaturable absorption region 105. Electrodes 106 and 107 are arranged onthe gain region 104 and the saturable absorption region 105,respectively. A region between the electrodes 106 and 107 is a currentnon-injection region (an electrode separation region) 108. An electrode109 is arranged on a back surface of the laser chip 100, that is, a backsurface of the semiconductor substrate 102.

In the two-electrode laser diode with the above-described configuration,when a forward bias voltage is applied between the electrodes 106 and109 arranged in upper and lower sections of the gain region 104,respectively, to inject a direct current, laser oscillation isperformed. Moreover, when a reverse bias voltage is applied between theelectrodes 107 and 109 arranged in upper and lower sections of thesaturable absorption region 105, respectively, self-pulsation operationis performed.

SUMMARY OF THE INVENTION

As described above, in an optical disk device, a reduction in coherenceof laser light is essentially desired. However, according to studies bythe inventors of the present invention, in the above-describedtwo-electrode laser diode in related art, even if self-pulsationoperation is performed, the coherence of laser light is not sufficientlyreduced. Thereby, the power region of available laser light or thedesign of optical disks are limited, so it is practically difficult toapply the above-described two-electrode laser diode in related art to alight source of the optical disk device.

It is desirable to provide a laser diode capable of performingself-pulsation operation, as well as capable of sufficiently reducingthe coherence of laser light and stably obtaining low-noise laser light.

It is also desirable to provide an optical disk device and an opticalpick-up using the above-described laser diode as a light source.

According to a first embodiment of the invention, there is provided alaser diode including: a laser chip including at least one laser stripewhich extends in a resonator length direction between a first endsurface and a second end surface opposed to each other, in which thelaser stripe includes a gain region and a saturable absorption region inthe resonator length direction, and the width of the laser stripe in thesaturable absorption region is larger than the width of the laser stripein the gain region.

According to a first embodiment of the invention, there is provided anoptical disk device including: a laser diode as a light source, in whichthe laser diode includes a laser chip including at least one laserstripe which extends in a resonator length direction between a first endsurface and a second end surface opposed to each other, the laser stripeincludes a gain region and a saturable absorption region in theresonator length direction, and the width of the laser stripe in thesaturable absorption region is larger than the width of the laser stripein the gain region.

According to a first embodiment of the invention, there is provided anoptical pick-up including: a laser diode as a light source, in which thelaser diode includes a laser chip including at least one laser stripewhich extends in a resonator length direction between a first endsurface and a second end surface opposed to each other, the laser stripeincludes a gain region and a saturable absorption region in theresonator length direction, and the width of the laser stripe in thesaturable absorption region is larger than the width of the laser stripein the gain region.

In the laser diode, the optical disk device and the optical pick-upaccording to the first embodiment of the invention, the length of thesaturable absorption region is typically, but not exclusively, smallerthan the length of the gain region.

As one typical example, the gain region is arranged on a side close tothe first end surface, and the saturable absorption region is arrangedon a side close to the second end surface. As another typical example,the saturable absorption regions are arranged on a side close the firstend surface and a side close to the second end surface, and the gainregion is arranged between the saturable absorption regions. As a stillanother example, the gain regions are arranged on a side close to thefirst end surface and a side close to the second end surface, and thesaturable absorption region is arranged between the gain regions. Thegain region and the saturable absorption region are typically arrangedadjacent to each other with a current non-injection region in between.

The gain region and the saturable absorption region are operableindependently of each other, and to do so, electrodes are arranged onthe gain region and the saturable absorption region, respectively, so asto be separated from each other. During operation of the laser diode,typically, a forward bias voltage is applied to the gain region toinject a direct current to the gain region, and if necessary, inaddition to the direct current, an alternating current or ahigh-frequency current is injected into the gain region. A reverse biasvoltage or a bias of 0 is applied to the saturable absorption region.

In the case where the laser stripe has a ridge shape, that is, in thecase where the laser stripe is a ridge stripe (a ridge waveguide), ifnecessary, the lateral refractive index step Δn of the laser stripe inthe gain region may be different from that in the saturable absorptionregion. For example, the lateral refractive index step Δn in the gainregion is larger than that in the saturable absorption region. In thiscase, the lateral refractive index step of the laser stripe means adifference between the refractive index in a laser stripe section andthe refractive index in sections on both sides of the laser stripe. Tohave different lateral refractive index steps Δn in the gain region andthe saturable absorption region, for example, the ridges in the gainregion and the saturable absorption region have different heights, andthe material of a dielectric film (an insulating film), a lightabsorbing film, a semiconductor film or the like formed on both sides ofthe ridges in the gain region is different from that in the saturableabsorption region.

The laser chip may have only one laser stripe or a plurality of laserstripes between the first end surface and the second end surface, andthe number of laser stripes is appropriately determined depending on theapplication of the laser diode or the like.

The laser chip includes a semiconductor layer (for example, an n-sidecladding layer, an active layer, a p-side cladding layer, a contactlayer or the like) forming a laser structure. The material of thesemiconductor layer forming the laser structure is not specificallylimited, and is appropriately selected depending on the wavelength oflight to be extracted from the laser diode. More specifically, a GroupIII-V compound semiconductor such as a GaN-based semiconductor, aGaAs-based semiconductor or a GaInP-based semiconductor, or a GroupII-VI compound semiconductor such as ZnSe may be used.

The optical disk device according to the first embodiment of theinvention includes an optical disk device for playing (reading) only, anoptical disk device for recording (writing) only, and an optical diskdevice for reproducing and recording, and any reproducing and/orrecording mode may be used. The optical disk device according to thefirst embodiment of the invention includes a reproducing optical systemand/or a recording optical system. The optical pick-up according to thefirst embodiment of the invention is suitably used in such an opticaldisk device.

According to a second embodiment of the invention, there is provided alaser diode including: a laser chip including at least one laser stripewith a ridge shape extends in a resonator length direction between afirst end surface and a second end surface opposed to each other, inwhich the laser stripe includes a gain region and a saturable absorptionregion in the resonator length direction, and a lateral refractive indexstep in the gain region is larger than a lateral refraction index stepin the saturable absorption region.

According to a second embodiment of the invention, there is provided anoptical disk device including: a laser diode as a light source, in whichthe laser diode includes at least one laser stripe with a ridge shapewhich extends in a resonator length direction between a first endsurface and a second end surface opposed to each other, the laser stripeincludes a gain region and a saturable absorption region in theresonator length direction, and a lateral refractive index step in thegain region is larger than a lateral refraction index step in thesaturable absorption region.

According to a a second embodiment of the invention, there is providedan optical pick-up including: a laser diode as a light source, in whichthe laser diode includes at least one laser stripe with a ridge shapewhich extends in a resonator length direction between a first endsurface and a second end surface opposed to each other, the laser stripeincludes a gain region and a saturable absorption region in theresonator length direction, and a lateral refractive index step in thegain region is larger than a lateral refraction index step in thesaturable absorption region.

In the laser diode, the optical disk device and the optical pick-upaccording to the second embodiment of the invention, in order for thegain region to have a larger lateral refractive index step than that inthe saturable absorption region, for example, the ridges in the gainregion and the saturable absorption region have different heights, andthe material of a dielectric film (an insulating film) a light absorbingfilm, a semiconductor film or the like formed on both sides of the ridgein the gain region is different from that in the saturable absorptionregion. The length of the saturable absorption region is typically, butnot exclusively, smaller than the length of the gain region.

As one typical example, the gain region is arranged on a side close tothe first end surface, and the saturable absorption region is arrangedon a side close to the second end surface. As another typical example,the gain region is arranged on each of sides close to the first endsurface and the second end surface, and the saturable absorption regionis arranged between the saturable absorption regions. As a still anotherexample, the saturable absorption region is arranged on each of sidesclose to the first end surface and the second end surface, and the gainregion is arranged between the saturable absorption regions. The gainregion and the saturable absorption region are typically arrangedadjacent to each other with a current non-injection region in between.

The gain region and the saturable absorption region are operableindependently of each other, and to do so, electrodes are included onthe gain region and the saturable absorption region, respectively, so asto be separated from each other. During operation of the laser diode,typically, a forward bias voltage is applied to the gain region toinject a direct current to the gain region, and if necessary, inaddition to the direct current, an alternating current or ahigh-frequency current is injected into the gain region. A reverse biasvoltage or a bias of 0 is applied to the saturable absorption region.

The laser chip may have only one laser stripe or a plurality of laserstripes between the first end surface and the second end surface, andthe number of laser stripes is appropriately determined depending on theapplication of the laser diode or the like.

Unless contrary to the nature of the invention, descriptions of thelaser diode, the optical disk device and the optical pick-up accordingto the first embodiment of the invention other than described above maybe applied to the laser diode, the optical disk device and the opticalpick-up according to the second embodiment of the invention.

As described above, in the laser diode, the optical disk device and theoptical pick-up according to the first embodiment of the invention andthe laser diode, the optical disk device and the optical pick-upaccording to the second embodiment of the invention, the laser stripeincludes the gain region and the saturable absorption region in theresonator length direction, thereby self-pulsation operation may beperformed. Moreover, when the width of the laser stripe in the saturableabsorption region is larger than the width of the laser stripe in thegain region, or when the lateral refractive index step in the gainregion is larger than the lateral refractive index step in the saturableabsorption region, during self-pulsation operation, the light density inthe gain region is higher than that in the saturable absorption region.In a region with high light density, a self-phase modulation effect as athird-order nonlinear optical effect is strongly produced. Thereby, thebroadening of a longitudinal-mode the light spectrum during theself-pulsation operation is increased to reduce the coherence time, sothe coherence of laser light is sufficiently reduced.

According to the first and second embodiments of the invention, a laserdiode capable of performing self-pulsation operation, and capable ofsufficiently reducing the coherence of laser light and stably obtaininglow-noise laser light is achievable. Moreover, when such a superiorlaser diode is used as a light source of an optical pick-up, ahigh-performance optical disk device is achievable.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a plan view and a sectional view illustrating atwo-electrode laser diode according to a first embodiment of theinvention.

FIG. 2 is a sectional view of the two-electrode laser diode taken alongline Y-Y of FIG. 1A.

FIG. 3 is a diagram illustrating changes in light density with the widthof a stripe.

FIGS. 4A and 4B are a plan view and a sectional view illustrating athree-electrode laser diode according to a second embodiment of theinvention.

FIGS. 5A and 5B are a plan view and a sectional view illustrating athree-electrode laser diode according to a third embodiment of theinvention.

FIGS. 6A and 6B are a plan view and a sectional view illustrating atwo-electrode laser diode according to a fourth embodiment of theinvention.

FIGS. 7A and 7B are sectional views illustrating the two-electrode laserdiode according to the fourth embodiment of the invention.

FIGS. 8A and 8B are sectional views illustrating the two-electrode laserdiode according to the fourth embodiment of the invention.

FIGS. 9A and 9B are a plan view and a sectional view illustrating atwo-electrode laser diode in related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments will be described in detail below referring to theaccompanying drawings.

First, a two-electrode laser diode according to a first embodiment ofthe invention will be described below.

FIGS. 1A and 1B illustrate the two-electrode laser diode, and FIG. 1A isa plan view, and FIG. 1B is a sectional view taken along line X-X (acentral line of a laser stripe) of FIG. 1A.

As illustrated in FIGS. 1A and 1B, the two-electrode laser diodeincludes a laser stripe 11 which extends throughout the length in aresonator length direction between a pair of parallel end surfaces 10 aand 10 b opposed to each other of a rectangular laser chip 10. The laserchip 10 includes a semiconductor layer 13 forming a laser structure onan electrically conductive semiconductor substrate 12. The semiconductorlayer 13 includes an active layer as well as an n-side cladding layer, ap-side cladding layer or the like (not illustrated). A section on a sideclose to the end surface 10 a of the laser stripe 11 is a gain region14, and a section on a side close to the end surface 10 b of the laserstripe 11 is a saturable absorption region 15. Electrodes 16 and 17 arearranged on the gain region 14 and the saturable absorption region 15,respectively. A region between the electrodes 16 and 17 is a currentnon-injection region (an electrode separation region) 18. An electrode19 is arranged on a back surface of the laser chip 10, that is, a backsurface of the semiconductor substrate 12.

The width of the laser stripe 11 is linearly increased from a width W₁on the end surface 10 a to a width W₂ (W₂>W₁) on the end surface 10 b ina direction from the end surface 10 a to the end surface 10 b of thelaser chip 10. Therefore, in this case, the width of the laser stripe 11in the saturable absorption region 15 is larger than the width of thelaser stripe 11 in the gain region 14. Moreover, the length of the gainregion 14 is larger than the length of the saturable absorption region15.

With regard to reflectivity of the end surfaces 10 a and 10 b, aheretofore known end surface coat film (not illustrated) is formed sothat the end surface 10 a has lower reflectivity than the reflectivityof the end surface 10 b.

For example, the dimensions of components of the two-electrode laserdiode are as follows, but the dimensions are not specifically limitedthereto.

Length of gain region 14 (length of electrode 16): 500 μm Length ofsaturable absorption region 15 (length of electrode 17): 20 μm Length ofcurrent non-injection region 18 (a space between electrodes 16 and 17):5 μm

Width W₁ of laser strip 11 on end surface 10 a: 1.4 μm

Width W₂ of laser stripe 11 on end surface 10 b: 3 μm

The material of the semiconductor layer 13 forming the laser structureis not specifically limited, and is appropriately selected according tothe central wavelength of laser light to be extracted from thetwo-electrode laser diode. More specifically, examples of the materialinclude a GaN-based semiconductor, a GaAs-based semiconductor, aGaInP-based semiconductor, a ZnSe-based semiconductor and the like.Moreover, the laser structure is not specifically limited, and any ofvarious heretofore known laser structures may be used.

An example of a sectional configuration vertical to the resonator lengthdirection of the laser chip 10 will be described below. In this case,the case where the semiconductor layer 13 is made of a GaN-basedsemiconductor, that is, the case where the two-electrode laser diode isa GaN-based laser diode will be described below. FIG. 2 is a sectionalview taken along line Y-Y of FIG. 1A, that is, an example of a sectionalview of the gain region 14. The sectional configuration of the saturableabsorption region 15 has the same configuration as that of the gainregion 14.

As illustrated in FIG. 2, in this example, the semiconductor layer 13 isformed by laminating an n-type AlGaN cladding layer 13 a, an n-type GaNlayer 13 b, for example, an undoped Ga_(1-x)In_(x)N (welllayer)/Ga_(1-y)In_(y)N (barrier layer, x>y) multiple quantum wellstructure active layer 13 c, a p-type AlGaN electronic barrier layer 13d, a p-type GaN layer 13 e, a p-type GaN/AlGaN superlattice claddinglayer 13 f and a p-type GaN contact layer 13 g in order on an n-type GaNsubstrate 20 as a semiconductor substrate 12. A ridge section is formedin an upper section of the p-type GaN/AlGaN superlattice cladding layer13 f and the p-type GaN contact layer 13 g, and the laser stripe 11 is aridge stripe.

For example, an insulating film 21 including a SiO₂ film and a Si filmarranged on the SiO₂ film is formed on a side surface of the laserstripe 11 and sections on both sides of the laser stripe 11 of thep-type GaN/AlGaN superlattice cladding layer 13 f. A p-side electrode 22is formed on the laser stripe 11 so as to electrically contact thep-type GaN contact layer 13 g. For example, but not exclusively, thep-side electrode 22 is made of Pd.

A pad electrode 23 is formed over the p-side electrode 22 and theinsulating film 21 so as to electrically contact the p-side electrode22. As the pad electrode 23, for example, an electrode with a Ti/Pt/Austructure is used, and the thicknesses of a Ti film, a Pt film and a Aufilm are, for example, but not exclusively, 15 nm, 50 nm and 300 nm,respectively. On the other hand, an n-side electrode 24 as the electrode19 is formed on a back surface of the n-type GaN substrate 20 so as toelectrically contact the n-type GaN substrate 20. As the n-sideelectrode 24, for example, an electrode with a Ti/Pt/Au structure isused, and the thicknesses of a Ti film, a Pt film and a Au film are, forexample, but not exclusively, 15 nm, 50 nm and 300 nm, respectively.

As an example of the thickness of the GaN-based semiconductor layerforming the laser structure, the n-type AlGaN cladding layer 13 a has athickness of 1200 nm, the n-type GaN layer 13 b has a thickness of 12nm, the well layer of the active layer 13 c has a thickness of 3.5 nm(with a well number of 3), the barrier layer of the active layer 13 chas a thickness of 7 nm, the p-type AlGaN electronic barrier layer 13 dhas a thickness of 10 nm, the p-type GaN layer 13 e has a thickness of12.3 nm, and the p-type GaN/AlGaN superlattice cladding layer 13 f has athickness of 400 nm. Moreover, the Al composition of the n-type AlGaNcladding layer 13 a is, for example, 0.05, the Al composition of thep-type AlGaN electronic barrier layer 13 d is, for example, 0.2, and theAl composition of the AlGaN layer of the p-type GaN/AlGaN superlatticecladding layer 13 f is, for example, 0.08.

FIG. 3 illustrates an example of changes in light intensity in the laserstripe during operation with the width of the laser stripe (a stripewidth) in the GaN-based laser diode. A lateral refractive index step Δn(a difference in the refractive index in a refractive-index profile inthe lateral direction) is 6×10⁻³. It is obvious from FIG. 3 that thesmaller the stripe width is, the more the light density increases.Therefore, it is clear that in the gain region 14 with a smaller widththan that of the saturable absorption region 15, the light density ishigher than that in the saturable absorption region 15.

Next, a method of manufacturing the two-electrode laser diode will bedescribed below referring to a GaN-based laser diode with a structureillustrated in FIG. 2 as an example of the two-electrode laser diode.

First, the n-type AlGaN cladding layer 13 a, the n-type GaN layer 13 b,the active layer 13 c, the p-type AlGaN electronic barrier layer 13 d,the p-type GaN layer 13 e, the p-type GaN/AlGaN superlattice claddinglayer 13 f and the p-type GaN contact layer 13 g are epitaxially grownin order on the n-type GaN substrate 20 by, for example, a metal organicchemical vapor deposition (MOCVD) method to form a structure.

Next, for example, an insulating film (not illustrated) such as a SiO₂film is formed on the whole surface of the p-type GaN contact layer 13g, and then the insulating film is patterned into a predetermined shapeby etching. Next, the patterned insulating film is used as an etchingmask, and the structure is etched until reaching the middle of a depthin a thickness direction of the p-type GaN/AlGaN superlattice claddinglayer 13 f by, for example, dry etching such as a reactive ion etching(RIE) method to form a ridge.

Next, for example, the SiO₂ film and the Si film are formed in order onthe whole surface while the insulating film used as the etching maskremains, and then sections above the laser stripe 11 of these films areselectively etched and removed. Thereby, the insulating film 21 isformed on the side surface of the ridge and sections on both sides ofthe ridge of the p-type GaN/AlGaN superlattice cladding layer 13 f.

Next, for example, as a material for forming the p-side electrode 22, aPd film in the same planar shape as that of the top surface of the ridgeis formed on the ridge, for example, by a liftoff method. Next, apredetermined section of the Pd film is etched and removed by an ionmilling method to form the current non-injection region 18, and to formthe p-side electrodes 22 on the gain region 14 and the saturableabsorption region 15.

Next, a resist pattern (not illustrated) with a stripe shape extendingto the current non-injection region 18 and an extension of the currentnon-injection region 18 is formed by lithography, and then a film forforming the pad electrode 23 is formed on the whole surface, forexample, by a vacuum deposition method. Then, the resist pattern isremoved together with the film formed on the resist pattern. Thereby,the pad electrodes 23 are formed on the p-side electrodes 22. Next, ifnecessary, the n-type GaN substrate 20 is polished from the back surfacethereof to reduce the thickness of the n-type GaN substrate 20 to apredetermined thickness. Then, the n-side electrode 23 is formed on theback surface of the n-type GaN substrate 20.

Next, the n-type GaN substrate 20 in which the laser structure is formedas described above is processed into a bar shape by cleavage or the liketo form the end surfaces 10 a and 10 b, and end surface coat films areformed on the end surfaces 10 a and 10 b by a heretofore knowntechnique, and then the bar is divided into chips. Thereby, the laserchip 10 is formed, and the desired two-electrode type GaN-based laserdiode is manufactured.

Next, operation of the two-electrode laser diode will be describedbelow.

In the gain region 14, a forward bias voltage is applied between theelectrodes 16 and 19 to inject a direct current, and in addition, ahigh-frequency voltage is applied to inject a high-frequency current (inthe case where high-frequency superposition is performed). In thesaturable absorption region 15, a reverse bias voltage or 0 V is appliedbetween the electrodes 17 and 19. Thus, the two-electrode laser diodeperforms self-pulsation operation.

In this case, the width of the laser stripe 11 in the gain region 14 issmaller than the width of the laser stripe 11 in the saturableabsorption region 15, so the light density in the gain region 14 ishigher than that in the saturable absorption region 15. Thereby, in thegain region 14, a self-phase modulation effect is strongly produced.Therefore, in the two-electrode laser diode, compared to a two-electrodelaser diode in related art, even if an equivalent light output is used,the broadening of a longitudinal-mode light spectrum duringself-pulsation operation is increased more. As a result, the coherencetime of laser light is reduced, so the coherence of the laser light islargely reduced, and the occurrence of feedback light noises whenreading information from an optical disk is prevented more effectively.

As described above, in the first embodiment, a two-electrode laser diodecapable of performing self-pulsation operation and capable ofsufficiently reducing the coherence of laser light and stably obtaininglow-noise laser light is achieved. Therefore, the two-electrode laserdiode is suitably used as a light source for an optical disk device,because the limit on the power region of available laser light or thedesign of optical disk is largely relaxed. A necessary amount ofreduction in coherence for the laser diode depends on the optical pathlength, the optical system or the like of the optical disk device.However, a larger amount of reduction in the coherence is desirable,because the laser diode is used for a larger number of kinds of opticaldisk devices.

Next, a three-electrode laser diode according to a second embodiment ofthe invention will be described below.

FIGS. 4A and 4B illustrate the three-electrode laser diode, and FIG. 4Ais a plan view, and FIG. 4B is a sectional view taken along line X-X (acentral line of a laser stripe) of FIG. 4A.

As illustrated in FIGS. 4A and 4B, the three-electrode laser diodeincludes the laser stripe 11 which extends throughout the length in aresonator length direction between a pair of parallel end surfaces 10Aand 10 b opposed to each other of the rectangular laser chip 10. Thelaser chip 10 includes the semiconductor layer 13 forming a laserstructure on the electrically conductive semiconductor substrate 12. Thesemiconductor layer 13 includes an active layer as well as an n-sidecladding layer, a p-side cladding layer or the like (not illustrated). Asection on a side close to the end surface 10 a and a section on a sideclose to the end surface 10 b of the laser stripe 11 are saturableabsorption regions 15 a and 15 b, respectively, and a section betweenthe saturable absorption regions 15 a and 15 b is the gain region 14.Electrodes 16, 17 a and 17 b are arranged on the gain region 14 and thesaturable absorption regions 15 a and 15 b, respectively. A regionbetween the electrodes 16 and 17 a is a current non-injection region 18a, and a region between the electrodes 16 and 17 b is a currentnon-injection region 18 b. The electrode 19 is arranged on the backsurface of the laser chip 10, that is, the back surface of thesemiconductor substrate 12.

The width of the laser stripe 11 in the gain region 14 is uniformthroughout the length of the gain region 14. The laser stripe 11 in eachof the saturable absorption regions 15 a and 15 b has a uniform width W₄(W₄>W₃) in a section from each of the end surfaces 10 a and 10 b to apoint at a distance L₁ from each of the end surfaces 10 a and 10 b, butthe width of the laser stripe 11 in each of the saturable absorptionregions 15 a and 15 b is linearly reduced from W₄ to W₃ in a sectionfrom the point at the distance L₁ to a point at a distance L₂ from eachof the end surfaces 10 a and 10 b. In other words, in this case, thewidth of the laser stripe 11 in the saturable absorption regions 15 aand 15 b is larger than that in the gain region 14. Moreover, the lengthof the gain region 14 is larger than the length of each of the saturableabsorption regions 15 a and 15 b.

The configuration other than described above of the three-electrodelaser diode is the same as that of the two-electrode laser diodeaccording to the first embodiment.

Moreover, a method of manufacturing the three-electrode laser diode isthe same as the method of manufacturing the two-electrode laser diodeaccording to the first embodiment.

Next, operation of the three-electrode laser diode will be describedbelow.

In the gain region 14, a forward bias voltage is applied between theelectrodes 16 and 19 to apply a direct current, and in addition, ifnecessary, a high-frequency voltage is applied to inject ahigh-frequency current (in the case where high-frequency superpositionis performed). In the saturable absorption regions 15 a and 15 b, areverse bias voltage or 0 V is applied between the electrodes 17 a and19 and between the electrodes 17 b and 19. Thus, the three-electrodelaser diode performs self-pulsation operation.

In this case, the width of the laser stripe 11 in the gain region 14 issmaller than the width of the laser stripe 11 in each of the saturableabsorption regions 15 a and 15 b, so the light density in the gainregion 14 is higher than that in the saturable absorption regions 15 aand 15 b. Thereby, in the gain region 14, the self-phase modulationeffect is strongly produced. Therefore, in the three-electrode laserdiode, as in the case of the two-electrode laser diode according to thefirst embodiment, the broadening of a longitudinal-mode light spectrumduring self-pulsation operation is increased. As a result, the coherencetime of laser light is reduced, so coherence of the laser light islargely reduced, and the occurrence of feedback light noises whenreading information from an optical disk is prevented more effectively.

In the second embodiment, the same advantages as those in the firstembodiment are obtained.

Next, a three-electrode laser diode according to a third embodiment ofthe invention will be described below.

FIGS. 5A and 5B illustrate the three-electrode laser diode, and FIG. 5Ais a plan view and FIG. 5B is a sectional view taken along line X-X ofFIG. 5A (a central line of a laser stripe).

As illustrated in FIGS. 5A and 5B, the three-electrode laser diodeincludes the laser stripe 11 which extends throughout the length in aresonator length diction between a pair of parallel end surfaces 10 aand 10 b opposed to each other of the rectangular laser chip 10. Thelaser chip 10 includes the semiconductor layer 13 forming a laserstructure on the electrically conductive semiconductor substrate 12. Thesemiconductor layer 13 includes an active layer as well as an n-sidecladding layer, a p-side cladding layer or the like (not illustrated). Asection on a side close to the end surface 10 a and a section on a sideclose to the end surface 10 b of the laser stripe 11 are gain regions 14a and 14 b, respectively, and a section between the gain regions 14 aand 14 b is the saturable absorption region 15. Electrodes 16 a, 16 band 17 are arranged on the gain regions 14 a and 14 b and the saturableabsorption region 15, respectively. A region between the electrodes 16 aand 17 is a current non-injection region 18 a, and a region between theelectrodes 16 b and 17 is a current non-injection region 18 b. Theelectrode 19 is arranged on the back surface of the laser ship 10, thatis, the back surface of the semiconductor substrate 12.

The laser stripe 11 in each of the gain regions 14 a and 14 b has auniform width W₃ throughout the length of each of the gain region 14 aand 14 b. The laser stripe 11 in the saturable absorption region 15 hasa uniform width W₄ (W₄>W₃) in a section between a point at a distance L₂from the end surface 10 a and a point at the distance L₂ from the endsurface 10 b, but the width of the laser stripe 11 in the saturableabsorption region 15 is linearly reduced from W₄ to W₃ in a section fromthe point at the distance L₂ to a point at a distance L₁ from each ofthe end surfaces 10 a and 10 b. In other words, in this case, the widthof the laser stripe 11 in the saturable absorption region 15 is largerthan that in each of the gain regions 14 a and 14 b. Moreover, thelength of each of the gain regions 14 a and 14 b is larger than thelength of the saturable absorption region 15.

The configuration other than described above of the three-electrodelaser diode is the same as that of the two-electrode laser diodeaccording to the first embodiment.

Moreover, a method of manufacturing the three-electrode laser diode isthe same as the method of manufacturing the two-electrode laser diodeaccording to the first embodiment.

Next, operation of the three-electrode laser diode will be describedbelow.

In the gain regions 14 a and 14 b, a forward bias voltage is appliedbetween the electrodes 16 a and 19 and between the electrodes 16 b and19 to apply a direct current, and in addition, if necessary, ahigh-frequency voltage is applied to inject a high-frequency current (inthe case where high-frequency superposition is performed). In thesaturable absorption region 15, a reverse bias voltage or 0 V is appliedbetween the electrodes 17 and 19. Thus, the three-electrode laser diodeperforms self-pulsation operation.

In this case, the width of the laser stripe 11 in each of the gainregions 14 a and 14 b is smaller than the width of the laser stripe 11in the saturable absorption region 15, so the light density in the gainregions 14 a and 14 b is higher than that in the saturable absorptionregion 15. Thereby, in the gain regions 14 a and 14 b, the self-phasemodulation effect is strongly produced. Therefore, in thethree-electrode laser diode, as in the case of the two-electrode laserdiode according to the first embodiment, the broadening of alongitudinal-mode light spectrum during self-pulsation operation isincreased. As a result, the coherence time of laser light is reduced, sothe coherence of the laser light is largely reduced, and the occurrenceof feedback light noises when reading information from an optical diskis prevented more effectively.

In the third embodiment, the same advantages as those in the firstembodiment are obtained.

Next, a two-electrode laser diode according to a fourth embodiment ofthe invention will be described below.

FIGS. 6A, 6B, 7A and 7B illustrate the two-electrode laser diode. FIG.6A is a plan view, and FIGS. 6B, 7A and 7B are sectional views takenalong lines X-X (a central line of a laser stripe), Y-Y and Z-Z of FIG.6A, respectively.

As illustrated in FIGS. 6A, 6B, 7A and 7B, the two-electrode laser diodeincludes the laser stripe 11 which extends throughout the length in aresonator length direction between a pair of parallel end surfaces 10 aand 10 b opposed to each other of the rectangular laser chip 10. In thiscase, the laser stripe 11 has a ridge shape. In other words, the laserstripe 11 is a ridge stripe. If necessary, a semiconductor layer or aninsulating film, for example, a SiO₂ film is arranged on sections onboth sides of the ridge.

The laser chip 10 includes the semiconductor layer 13 forming a laserstructure on the electrically conductive semiconductor substrate 12, anda ridge is formed in an uppermost section of the semiconductor layer 13.In FIG. 6B, a top surface of the semiconductor layer 13 in sections onboth sides of the ridge is indicated with a broken line. Thesemiconductor layer 13 includes an active layer AL as well as an n-sidecladding layer, a p-side cladding layer or the like (not illustrated).

A section on a side close to the end surface 10 a of the laser stripe 11is the gain region 14, a section on a side close to the end surface 10 bof the laser stripe 11 is the saturable absorption region 15. Theelectrodes 16 and 17 are arranged on the gain region 14 and thesaturable absorption region 15, respectively. A region between theelectrodes 16 and 17 is the current non-injection region 18. Theelectrode 19 is arranged on the back surface of the laser chip 10, thatis, the back surface of the semiconductor substrate 12.

The laser stripe 11 has a uniform width throughout the length thereof.Therefore, the width of the laser stripe 11 in the gain region 14 isequal to the width of the laser stripe 11 in the saturable absorptionregion 15.

In this case, the height of the ridge of the laser stripe 11 in the gainregion 14 is different from that in the saturable absorption region 15,and the height of the ridge in the gain region 14 is larger than that inthe saturable absorption region 15. In other words, as a distancebetween a top surface of the semiconductor layer 13 and a top surface ofthe active layer AL in sections on both sides of the laser stripe 11,the gain region 14 has a distance D₁ which is smaller than a distance D₂in the saturable absorption region 15.

For example, in the case where the two-electrode laser diode is aGaN-based laser diode, as illustrated in FIGS. 8A and 8B, for example, aSiO₂ film and an insulating film 21 made of a Si film are formed on aside surface of the ridge and sections on both sides of the ridge of thesemiconductor layer 13.

The configuration other than described above of the two-electrode laserdiode is the same as that of the two-electrode laser diode according tothe first embodiment.

Moreover, a method of manufacturing the two-electrode laser diode is thesame as the method of manufacturing the two-electrode laser diodeaccording to the first embodiment.

Next, operation of the two-electrode laser diode will be describedbelow.

In the gain region 14, a forward bias voltage is applied between theelectrodes 16 and 19 to apply a direct current, and in addition, ifnecessary, a high-frequency voltage is applied to inject ahigh-frequency current (in the case where high-frequency superpositionis performed). In the saturable absorption region 15, a reverse biasvoltage or 0 V is applied between the electrodes 17 and 19. Thus, thetwo-electrode laser diode performs self-pulsation operation.

In this case, as the distance between the top surface of thesemiconductor layer 13 and the top surface of the active layer AL insections on both sides of the laser stripe 11, as described above, thegain region 14 has the distance D₁ which is smaller than the distance D₂in the saturable absorption region 15. Thereby, a lateral refractiveindex step Δn in the gain region 14 is larger than that in the saturableabsorption region 15. Therefore, optical confinement in the gain region14 is stronger than that in the saturable absorption region 15. As aresult, the light density in the gain region 14 is higher than that inthe saturable absorption region 15, thereby in the gain region 14, theself-phase modulation effect is strongly produced. Therefore, in thetwo-electrode laser diode, as in the case of the two-electrode laserdiode according to the first embodiment, the broadening of alongitudinal-mode light spectrum during self-pulsation operation isincreased. As a result, the coherence time of laser light is reduced, sothe coherence of the laser light is largely reduced, and the occurrenceof feedback light noises when reading information from an optical diskis prevented more effectively.

In the fourth embodiment, the same advantages as those in the firstembodiment are obtained.

Next, a two-electrode laser diode according to a fifth embodiment of theinvention will be described below.

The fifth embodiment is a combination of the first embodiment and thefourth embodiment.

More specifically, in the two-electrode laser diode according to thefirst embodiment, as in the case of the two-electrode laser diodeaccording to the fourth embodiment, the gain region 14 has the distanceD₁ which is smaller than the distance D₂ in the saturable absorptionregion 15, thereby the two-electrode laser diode according to the fifthembodiment is formed. Thus, the light density in the gain region 14becomes higher, and the self-phase modulation effect is produced morestrongly.

In the fifth embodiment, the same advantages as those in the firstembodiment are obtained.

Next, a three-electrode laser diode according to a sixth embodiment willbe described below.

The sixth embodiment is a combination of the second embodiment and thefourth embodiment. More specifically, in the three-electrode laser diodeaccording to the second embodiment, as in the case of the two-electrodelaser diode according to the fourth embodiment, the gain region 14 hasthe distance D₁ which is smaller than the distance D₂ in the saturableabsorption regions 15 a and 15 b, thereby the three-electrode laserdiode according to the sixth embodiment is formed, Thus, the lightdensity in the gain region 14 becomes higher, and the self-phasemodulation effect is produced more strongly.

In the sixth embodiment, the same advantages as those in the firstembodiment are obtained.

Next, a three-electrode laser diode according to a seventh embodiment ofthe invention will be described below.

The seventh embodiment is a combination of the third embodiment and thefourth embodiment. More specifically, in the three-electrode laser diodeaccording to the third embodiment, as in the case of the two-electrodelaser diode according to the fourth embodiment, the gain regions 14 aand 14 b each have the distance D₁ which is smaller than the distance D₂in the saturable absorption region 15, thereby the three-electrode laserdiode according to the seventh embodiment is formed. Thus, the lightdensity in the gain regions 14 a and 14 b becomes higher, and theself-phase modulation effect is produced more strongly.

In the seventh embodiment, the same advantages as those in the firstembodiment are obtained.

Although the invention is specifically described referring to theabove-described embodiments, the invention is not limited thereto, andmay be variously modified.

For example, the values, the configurations, the shapes, the substrates,the processes and the like described in the embodiments are onlyexamples, and any other values, configurations, shapes, substrates,processes and the like may be used.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2008-189265 filedin the Japanese Patent Office on Jul. 23, 2008, the entire content ofwhich are hereby incorporated by references.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A laser diode comprising: a laser chip includingat least one laser stripe which extends in a resonator length directionbetween a first end surface and a second end surface opposed to eachother, wherein, the laser stripe includes a gain region and a saturableabsorption region in the resonator length direction, the width of thelaser stripe in the saturable absorption region is larger than the widthof the laser stripe in the gain region, the length of the saturableabsorption region is smaller than the length of the gain region, and thesaturable absorption regions are arranged on a side close to the firstend surface and a side close to the second end surface, and the gainregion is arranged between the saturable absorption regions,
 2. A laserdiode comprising: a laser chip including at least one laser stripe whichextends in a resonator length direction between a first end surface anda second end surface opposed to each other, wherein, the laser stripeincludes a gain region and a saturable absorption region in theresonator length direction, the width of the laser stripe in thesaturable absorption region is larger than the width of the laser stripein the gain region, the length of the saturable absorption region issmaller than the length of the gain region.the gain regions are arrangedon a side close to the first end surface and a side close to the secondend surface, and the saturable absorption region is arranged between thegain regions.
 3. A laser diode comprising: a laser chip including atleast one laser stripe which extends in a resonator length directionbetween a first end surface and a second end surface opposed to eachother, wherein, the laser stripe includes a gain region and a saturableabsorption region in the resonator length direction, the width of thelaser stripe in the saturable absorption region is larger than the widthof the laser stripe in the gain region, and the laser stripe has a ridgeshape, and the lateral refractive index step of the laser stripe in thegain region is different from that in the saturable absorption region.4. A laser diode comprising: a laser chip including at least one laserstripe with a ridge shape which extends in a resonator length directionbetween a first end surface and a second end surface opposed to eachother, wherein, the laser stripe includes a gain region and a saturableabsorption region in the resonator length direction, and a lateralrefractive index step in the gain region is larger than a lateralrefraction index step in the saturable absorption region.
 5. An opticaldisk device comprising: a laser diode as a light source, wherein, thelaser diode includes at least one laser stripe with a ridge shape whichextends in a resonator length direction between a first end surface anda second end surface opposed to each other, the laser stripe includes again region and a saturable absorption region in the resonator lengthdirection, and a lateral refractive index step in the gain region islarger than a lateral refraction index step in the saturable absorptionregion.
 6. An optical pick-up comprising: a laser diode as a lightsource, wherein, the laser diode includes at least one laser stripe witha ridge shape which extends in a resonator length direction between afirst end surface and a second end surface opposed to each other, thelaser stripe includes a gain region and a saturable absorption region inthe resonator length direction, and a lateral refractive index step inthe gain region is larger than a lateral refraction index step in thesaturable absorption region.